1. Field of the Invention
The present invention relates to a semiconductor light receiving element.
2. Related Background of the Invention
In the field of optical communications, wavelength components of light in 1.3 xcexcm and 1.55 xcexcm bands are used as signal light. In a certain optical communication line, light of wavelength components in the 1.3 xcexcm and 1.55 xcexcm bands is transmitted in a single optical fiber.
In such an optical communication system, a light receiving element needs to selectively receive signal light having one component of these wavelength components. The light receiving element is required to block light of a wavelength component in the 1.3 xcexcm band and to receive light of a wavelength component in the 1.55 xcexcm band. In order to realize the wavelength selection, a single InGaAsP semiconductor layer is provided between a semiconductor substrate and a light absorbing layer. The InGaAsP semiconductor layer absorbs light of a wavelength component in the 1.3 xcexcm band and transmits light of a wavelength component in the 1.55 xcexcm band. In order to realize sufficient absorption of light in the 1.3 xcexcm band, the thickness of the InGaAsP semiconductor layer is approximately several micrometers. The thickness of the single InGaAsP semiconductor layer having an absorption coefficient, xcex1=8500 cmxe2x88x921, and an absorption edge of wavelength, xcexg=1.44 xcexcm, is estimated to be approximately 5.5 xcexcm.
There is miscibility gap in the phase diagram of InGaAsP semiconductor. The phase separation is apt to occur when a thick InGaAsP semiconductor film is grown. According to the above estimation, however, the film thickness of several micrometers is required for obtaining sufficient filter characteristics. In order to obtain a film of such a thickness, an InGaAsP semiconductor film is grown under strict control of film growth conditions.
Although it is difficult to grow an InGaAsP semiconductor film having a thickness that realizes desired filter characteristics, the realization of desired filter characteristics is required for a semiconductor light receiving element having an InGaAsP semiconductor film.
Accordingly, it is an object of the present invention to provide a semiconductor light receiving element having a structure capable of realizing desired filter characteristics.
One aspect of the present invention relates to a semiconductor light receiving element. The semiconductor light receiving element comprises a light incident face, a light detecting portion, and a first filter portion. The light detecting portion has a light absorbing layer containing III-V semiconductor. The first filter portion is provided between the light incident face and the light detecting portion. The first filter portion has a plurality of III-V semiconductor layers and a plurality of InGaAsP semiconductor layers. The III-V semiconductor layers and the InGaAsP semiconductor layers are arranged alternately.
Since the first filter portion includes the plurality of InGaAsP semiconductor layers, desired filter characteristics can be realized by the total thickness of these semiconductor layers.
The semiconductor light receiving element may further comprise a substrate provided between the light incident face and the light detecting portion. Incident light can be transmitted through the substrate. Thus, a back illuminated type semiconductor light receiving element is provided.
There are a number of embodiments of the semiconductor light receiving element as follows. In one embodiment, the first filter portion may be provided between the light detecting portion and the substrate. The first filter portion can also block noise light entering from a side face of the substrate. In another embodiment, the first filter portion may be provided between the substrate and the light incident face. The substrate is provided between the first filter portion and the light detecting portion, thus lowering the possibility that electron-hole pairs generated in the first filter portion reach the light detecting portion. In still another embodiment, the semiconductor light receiving element may further comprises a second filter portion. The second filter portion has a plurality of III-V semiconductor layers and a plurality of InGaAsP semiconductor layers. The plurality of III-V semiconductor layers and the plurality of InGaAsP semiconductor layers are arranged alternately in a direction of a predetermined axis. The substrate is provided between the first and second filter portions.
In the semiconductor light receiving element, a thickness of each InGaAsP semiconductor layer is preferably 1.5 xcexcm or less. Each InGaAsP semiconductor layer has s reduced thickness, thus making it possible to reduce the occurrence of phase separation.
In the semiconductor light receiving element, the number of InGaAsP semiconductor layers is preferably five or more, thereby making the desired filter characteristics easy to obtain.
In the semiconductor light receiving element, each III-V semiconductor layer in the first filter portion may include an InP semiconductor layer. Preferably, the thickness values of the respective InGaAsP semiconductor layers differ from each other, whereby the first filter portion does not exhibit any periodicity coming from the arrangement of these InGaAsP semiconductor layers.
In the semiconductor light receiving element, each III-V semiconductor layer in the first filter portion preferably includes an InP semiconductor layer. The thickness values of the InP semiconductor layers are smaller than those of the InGaAsP semiconductor layers.
In the semiconductor light receiving element, each III-V semiconductor layer in the first filter portion may include an InP semiconductor layer. The InP semiconductor layers and the InGaAsP semiconductor layers in the first filter portion have their respective thickness values so that the first filter portion has a light transmission window in a wavelength range from 1.45 xcexcm to 1.65 xcexcm inclusive.
The semiconductor light receiving element may further comprise an InP window layer. The detecting portion is provided between the InP window layer and the substrate. The light detecting portion includes a semiconductor region having a conductivity type different from that of the light absorbing layer. This semiconductor region and the light absorbing layer are provided to constitute a junction.
The above object and other objects, features, and advantages of the present invention will become more easily apparent from the following detailed description of a preferred embodiment of the present invention which proceeds with reference to the accompanying drawings.